1. Field of the Invention
This invention relates to scanning electron microscopes and, more particularly, to a scanning electron microscope well suited for using a secondary electron detector of an absorption current scheme to absorb the ions generated by electron-beam irradiation in a state of high sample chamber pressure.
2. Description of the Related Art
It is a conventional practice to use an absorption current scheme in the scanning electron microscope having a variable sample chamber pressure wherein, in a state of high sample chamber pressure, the secondary electrons generated at a sample surface are accelerated by a positive bias electrode positioned in the above and bombarded against the gas molecules remaining in the sample chamber to generate ions so that the ions are absorbed at a sample table or stage positioned within the sample chamber to amplify and A/D-convert an absorption current signal thereby forming an image. Because the ions have secondary electron information, it is possible to obtain surface information of the sample. The positive bias voltage positioned above can be varied, with a 10-V step, from 0V to 300V. By changing the speed for accelerating the secondary electrons caused from the sample surface, it is possible to adjust the amount of ion generation. The generated ions are moved back toward the sample by an electric field formed between the sample and the bias voltage to be absorbed at the conductive sample table or stage positioned within the sample chamber.
In the meanwhile, it has been revealed that, in the conventional scanning electron microscope of an absorption current scheme, there is encountered a problem the amount of ion absorption decreases when an insulating sample in observation is large in size. Making consideration on the cause, it has been thought of forming a factor that the potential on a sample itself changes depending on an insulating sample size to cause a disturbance in the electric field formed between the bias electrodes thus changing the mobility of the ions within the sample chamber.
It is an object of the present invention to provide a scanning electron microscope capable of efficiently detecting ions, such as primary electron excitation ions, reflection electron excitation ions or secondary electron excitation ions caused by a bias electric field, thereby obtaining an absorption current.
(1) In order to achieve the foregoing object, the present invention is provides a scanning electron microscope for irradiating an electron beam to a sample while keeping a sample chamber pressure at 1 Pa or higher to detect an generated ion and display a sample image, comprising an ion detecting electrode exclusive for detecting the ion.
This structure makes it possible to efficiently detect ions, such as primary electron excitation ions, reflection electron excitation ions or secondary electron excitation ions caused by a bias electric field, thereby obtaining an absorption current.
(2) In the above (1), preferably, the ion detecting electrode is arranged nearby a path for accelerating an ion by a bias electrode.
(3) In the above (2), preferably, the ion detecting electrode is fixed on and electrically connected to a sample table for holding the sample or to a sample stage supporting the sample table, the ion detecting electrode being variable in vertical length, further comprising control means for controlling such that a distance between an upper end of the ion detecting electrode and the bias electrode is constant.
(4) In the above (2), preferably, the ion detecting electrode is arranged distant from a sample table for holding the sample or from a sample stage supporting the sample table such that a distance between the bias electrode and an upper end of the ion detecting electrode is constant, the ion detecting electrode being variable in vertical length, further comprising control means for controlling such that a distance between a lower end of the ion detecting electrode and the sample is constant.
(5) In the above (4), preferably, voltage applying means is further provided for applying a voltage to the ion detecting electrode.